A process for preparing the above-mentioned styrenic resins in its patent application Ser. No. 24,837 filed on Mar. 28, 1979 which is a continuation-in-part of patent application Ser. No. 916,576, now abandoned filed on June 19, 1978, which is a continuation-in-part of patent application Ser. No. 890,211, now abandoned filed on Mar. 27, 1978, has previously been disclosed in which the rubber is polymerized in situ. According to the earlier process, a monomer mixture capable of forming a rubbery polymer by polymerization is added to a rigid polymeric matrix, generally a styrene acrylonitrile or an acrylonitrile-butadiene-styrene resin, said monomer mixture comprising an alkyl ester of acrylic acid and a cross-linking agent, such as conjugated diolefins. In that process the monomer mixture is absorbed by the rigid polymeric matrix and is then suspension polymerized to form in situ a rubbery polymer dispersed in the polymeric matrix.
Although the above described process allows one to prepare ASA or ABSA resins having valuable physical properties, it has been found that the simultaneous introduction into the suspension of the acrylic monomer together with the crosslinking agent, i.e. the conjugated diolefin, can lead in some cases to a side reaction, with formation of a Diels-Adler addition product between the acrylic monomer and the conjugated diolefin. The presence of that addition compound in the final product can have a detrimental influence on the physical properties of the resin. On the other hand, it has also been noticed that during the absorption period, which is generally carried out in the absence of oxygen, some amount of acrylic monomer can polymerize because the temperature is relatively high during the absorption step. However, this high temperature is necessary to obtain a homogeneous distribution of the acrylic monomer into the polymeric matrix. The consequence of such a polymerization during the absorption step is the formation of a non-homogeneous rubbery copolymer, which does not contain unsaturated units distributed along the polymeric chain. However, the presence of the unsaturated units distributed along the polymeric chain is necessary to form cross-linked domains at the end of the reaction. Such an early polymerization can occur even if the acrylic monomer contains a usual polymerization inhibitor, such as hydroquinone methyl ether, which is generally added to avoid the polymerization during transport or storage of the monomer.